1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission, and more particularly to a hydraulic control system which selectively applies and releases a plurality of friction coupling elements of an automatic transmission.
2. Description of the Prior Art
Generally an automatic transmission comprises a torque converter and a multi-speed transmission gear mechanism consisting of, for instance, planetary gears. The transmission gear mechanism has a plurality of friction coupling elements such as clutches and brakes for changing power transmission paths in the gear mechanism and the friction coupling elements are caused to engage or disengage by a hydraulic control system.
In an electronic control automatic transmission having an electronic control unit, gear-shifting is effected according to an upshift map in which 1-2 upshift line, 2-3 upshift line and 3-4 upshift line are set using throttle opening and turbine speed (or vehicle speed) as parameters and a downshift map in which 2-1 downshift line, 3-2 downshift line and 4-3 downshift line are set using throttle opening and turbine speed (or vehicle speed) as parameters. The upshift map and the downshift map are stored in a built-in memory in the control unit.
If the line pressure of the hydraulic control system upon a gear-shifting is too high, the friction coupling elements are abruptly engaged and shift shock is generated and if the line pressure is too low, gear-shifting requires a long time and the friction coupling elements wear abnormally or generate heat.
In order to avoid such problems, there has been proposed a hydraulic control system as disclosed, for instance, in Japanese Patent Publication No. 61(1986)-148021 in which the line pressure upon a gear-shifting is set according to the engine load (as represented, for instance, by the throttle opening) and the kind of gear-shifting (e.g., from which gear speed to which gear speed). In the system, a map such as shown in FIG. 12 is prepared and the line pressure PLo is set with reference to the map.
However when the line pressure PLo is set in such a manner, the following problem arises in the case of a backout shift, i.e., an upshift due to reduction of the engine load. That is, when the amount of depression of the accelerator pedal is reduced so that the throttle opening is reduced from a value a to a value b as shown by arrow x in FIG. 13, whereby the operating condition determined on the basis of the relation between the throttle opening and the turbine speed moves across the upshift line shown in FIG. 13 to a range where the transmission should upshift, the line pressure PLo determined according to the map shown in FIG. 12 is equal to that which should be selected when the operating condition moves across the upshift line as shown by arrow y at a point c the throttle opening at which is equal to the value b. The upshift to be made when the operating condition moves in the manner shown by the arrow x must be made at a turbine speed higher than that at which the upshift is made when operating condition moves in the manner shown by the arrow y. Since the change in inertia of the transmission and the engine becomes larger as the turbine speed increases, the line pressure PLo determined according to the map shown in FIG. 12 is too low for the former upshift and the upshifting time is elongated.
In order to overcome this problem, in the control system disclosed, for instance, in Japanese Unexamined Publication of Translated version 58(1983)-501477, the line pressure is set on the basis of the engine torque and the engine speed at the time the gear-shifting is to be made.
However, since the control unit for controlling the automatic transmission is generally separate from the control unit for controlling the engine, in order for the control unit for the automatic transmission to obtain data on the engine torque, the control units must be connected through many signal lines, and at the same time, a lot of controlling data based on various basic experiments are required. Thus the system disclosed in the publication identified above is difficult to put into practice though it can provide a relatively fine control.